Multilayer ceramic capacitors consist of a plurality of interleaved and staggered layers of an electrically conductive film of metal (termed "electrode"), formed by the deposition (usually by screen printing or variations thereof) of a thick film paste (termed an "electrode composition") and electrically insulating layers of a ceramic oxide (termed "dielectric"), formed by laying a cast dielectric tape or by casting a dielectric slurry over the dried electrode. Such capacitors are well known in the art. U.S. Pat. No. 2,389,420, for example, describes the structure, manufacture and properties of monolithic multilayer ceramic capacitors (MLCs).
The electrode composition is usually a dispersion of finely divided precious metal powders such as palladium, silver, gold or platinum or their mixtures in a vehicle which is usually solely organic in nature. Dispersions of non-precious metals such as copper and nickel have also been shown to have utility in electrode compositions. The vehicle or organic medium is usually composed of a mixture of a polymeric resin which imparts viscosity to the composition and appropriate solvents for processing compatibility, particularly with respect to drying. Other organic materials are usually added to the vehicle to control paste rheology. Typical electrode composition metal concentrations range from 40 to 70% by weight, with the remainder being vehicle. Electrode compositions are deposited, usually by screen printing techniques, on dried dielectric layers, then dried to remove solvents and leave a mixture of metal powders and resin.
The dielectric layer is usually composed of finely divided oxide powders dispersed in a resin. Barium titanate (BaTiO.sub.3) and other oxides such as neodymium titanate (Nd.sub.2 Ti.sub.2 O.sub.7) strontium titanate (SrTiO.sub.3) and magnesium titanate (MgTiO.sub.3) are used. Additions are usually made to these oxides to control various electrical characteristics, particularly to maximize dielectric constant (K) while controlling the temperature dependence of dielectric constant and insulation resistance, among other properties. The resin is present in the dielectric layers to facilitate handling during manufacture.
Multilayer ceramic capacitors are manufactured by building up an interleaved configuration of electrode and dielectric layers, dicing individual parts out of the build-up, subjecting the parts to a slow burnout and then high temperature firing. Burnout is done to remove the organic resin in the electrode and dielectric layers to avoid rapid outgassing and rupture of the parts. Firing is done to a peak temperature (the "dielectric maturation temperature") both to densify the dielectric for maximum dielectric constant and physical strength and to react the chemical constituents of the dielectric so that other desired electrical characteristics are achieved. During the firing step, the powder grains in the electrode layers also sinter and densify to produce a continuous, highly electrically conductive metal film.
A principal problem in multilayer ceramic capacitor manufacture originates from the firing of the electrode and dielectric films simultaneously. During firing, a common physical defect termed a "delamination" is formed. Delaminations are defined as separations of the electrode and dielectric layers which cause a void in what is meant to be a monolithic solid body. Delaminations are detrimental to finished capacitor performance because they can cause electrodes of opposite polarity to contact and form an electrical short. Delaminations can also trap liquids used in subsequent processing. These liquids can leave electrical charge carriers in the delamination voids and thus reduce the insulation resistance of the finished part. For high parts yields and maximum reliability of the finished parts, delaminations must be closely controlled and preferably eliminated entirely.
The dielectric maturation temperature determines the metal powders used in the electrode. The metals are chosen to have melting points above the dielectric maturation temperature to avoid melting the electrode during firing and the formation of discontinuous fired metal films. For example, dielectrics which mature at 1350.degree. C. require use of electrode compositions which contain only palladium, gold or platinum powders or their mixtures because of the high melting point of these metals and their alloys. Lower firing dielectrics, such as those which fire at 1100.degree. C., typically permit use of electrode compositions which contain mixtures of palladium and silver in the weight ratio of 30%:70%. Dielectrics which can be fired in non-oxidizing atmospheres permit use of non-precious metal electrodes. Mixtures of Pd and Ag are used with low firing dielectric compositions since Ag is less expensive than Pd and the lower dielectric maturation temperature allows Ag to be used. Typically Pd/Ag electrodes begin to sinter before the dielectric layers sinter and a shrinkage mismatch takes place which usually leads to delaminations in the finished MLC part. The same phenomenon, however, also occurs when 100% Pd electrodes are used on higher firing dielectric bodies.
One method of sintering shrinkage mismatch control common to the MLC industry is the incorporation of inorganic oxides into the electrode composition to inhibit, or move the temperature of rapid sintering shrinkage of the Pd and Pd/Ag powders to higher temperatures. These inorganic oxides upon firing must be non-reducing in the Pd or Pd/Ag metals because, if a reaction between the Pd or Pd/Ag results in the reduction of the metal oxide to its metal moiety, that metal moiety can act as a low temperature flux to promote the sintering of the Pd or Pd/Ag, thereby aggravating the mismatch of the sintering shrinkage of the electrode and dielectric. In addition, in order for an oxide to act as a sintering inhibitor, the oxide must also be insoluble in the Pd or Pd/Ag since a soluble oxide can also act to promote sintering and subsequent densification.
Usually an oxide ceramic powder which will not be detrimental to the dielectric layers is chosen as the sintering inhibitor. Typical examples include barium titanate, aluminum oxide, silicon oxide, etc. A principal problem with use of a separate oxide is that it must be well dispersed throughout the electrode paste in order to be effective. Standard milling techniques for dispersing a precious metal powder in a viscous paste vehicle, however, are usually not adequate to disperse the oxide additives. To achieve adequate additive dispersion, a high amount of dispersion energy must be used, but this will also tend to deform the precious metal grains to the point where ultimate performance of the electrode paste is degraded. For this reason, another method of intimately mixing the oxide sintering inhibitor with the precious metal powder is required. An additional constraint on any oxide additions to the electrode for the purpose of inhibiting sintering shrinkage is that the amount of oxide used must be as low as possible. Incorporating oxides into the fired layers of electrode can cause decreased electrical conductivity of the electrode layer, leading to higher electrical losses and finished MLCs which do not meet specifications. This is particularly important for capacitors which are used at high applied frequencies where electrode electrical conductivity is an important factor in overall electrical loss of the part.
Pepin, in U.S. Pat. No. 4,954,926, describes a method in which the sintering inhibitor is added to the organic medium as a liquid metal resinate. The previously discussed restrictions on the metal oxide products of the decomposition of these resinates apply in order effectively to inhibit the sintering of the Pd or Pd/Ag. By adding metal resinates, a particularly good dispersion of the metal oxide sintering inhibitor is achieved.
This invention is directed to yet another effective means of incorporating metal oxide sintering inhibitors into MLC electrodes. For the purposes of the discussion and the examples, the metallurgies used in the MLC electrode compositions will be exemplified by Pd and Pd/Ag, but the technology disclosed is applicable to other metallurgies including both the precious metals and non-precious metals which could be used in MLC electrodes.